A query execution plan may specify a sequence of steps for processing a query. The steps may be performed by upper-layer operations in network nodes of a distributed database system. Some of the steps specified by a query execution plan may require the network nodes to exchange data messages. For example, a step may require a join operation of data tables maintained at different network nodes, and consequently information from some of the tables may be communicated between the network nodes via data messages. Network nodes may maintain buffers for storing and processing data messages. For example, each network node may maintain a communication buffer for storing incoming data messages, as well as multiple working buffers for storing data messages that are being processed, or available to be processed, by upper-layer operations.